Recombinant adenoviral vectors have been used in the development of vaccines. To date, approximately 55 different adenovirus serotypes have been identified. The subgroup C adenoviruses have been most extensively studied for applications such as vaccination and gene therapy. Adenovirus serotypes 2 and 5 (Ad2 and Ad5), in particular, are widely used in the field. Importantly, Ad5 vector-based vaccines have been shown to elicit potent and protective immune responses in a variety of animal models. Moreover, large-scale clinical trials for HIV vaccination using Ad5-based recombinant vectors are ongoing (see, e.g., WO 01/02607; WO 02/22080; Shiver et al., Nature. 415:331-335, 2002; Letvin et al., Annu. Rev. Immunol. 20:73-99, 2002; and Shiver and Emini, Annu. Rev. Med. 55:355, 2004).
The usefulness of recombinant Ad5 vector-based vaccines for HIV and other pathogens, however, may be limited due to high pre-existing anti-Ad5 immunity in human populations. The presence of anti-Ad5 immunity has been correlated with a reduction in the immunogenicity of Ad5-based vaccines in studies in mice and rhesus monkeys. Early data from phase-1 clinical trials show that this problem may also occur in humans. Although both Ad5-specific neutralizing antibodies (NAbs) and CD8+ T lymphocytes contribute to anti-Ad5 immunity, the Ad5-specific NAbs appear to play the primary role in this process (Sumida et al., J. Virol., 174:7179-7185, 2004).
Accordingly, there is an unmet need in the field for alternative adenoviral vectors that have low seroprevalence and potent immunogenicity.